Post on 21-Jan-2016
HEAT TRANSFER
Instructor: M.T. Esfidani
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Heat transfer
• Heat transfer is the exchange of thermal energy between physical systems, depending on the temperature and pressure, by dissipating heat.
• The fundamental modes of heat transfer are:• conduction
• convection • radiation.
• Heat transfer always occurs from a region of high temperature to another region of lower temperature.
• Thermal equilibrium is reached when all involved bodies and
the surroundings reach the same temperature.
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Definitions Mechanisms of Heat (Thermal Energy) Transfer:
Conduction: transmission of heat across matter, due to direct physical contact, e.g. in solids, liquids and gases.
Radiation: heat transfer due to electromagnetic radiation across a space, even in a vacuum.
Convection: heat transfer by “currents” in a gas or liquid, due to temperature differences or forced flow, an important mechanism of energy transfer between a solid surface and a liquid or a gas.
(Source of illustrations: http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html)
All mechanisms may be involved in practical heat transfer, but the dominant mechanism differs in different cases. 3
Definitions
Heat CAPACITY (of a system): Quantity of ENERGY required to heat a whole system by 1 K (Kelvin). Unit: J·K-1
Concepts & Terminologies:
SPECIFIC HEAT (or SPECIFIC HEAT CAPACITY): Quantity of ENERGY required to heat a unit mass of a system by 1 K. Unit: J·kg-1·K-1
Kelvin = Degree Celsius (oC) + 273
J= joule4
Definitions
( note: 1W = 1 J s-1)
THERMAL CONDUCTIVITY, k : a measurement of heat flow through a body. It is the heat transmitted in unit time, in a direction normal to a surface of unit area, through a distance, d, across a unit temperature difference over the distance.
Unit: Wm-1K-1 (or Wm-1 °C-1) (error in handout “m-3” )
(analogous to electrical conductivity or hydraulic permeability)
T1
T2
Q A
d
Temperature profile
(Eq. 6a)(Eq. 6a)Q, heat flow per unit time (Js-1) × d, distance (m)
A, area (m2) × (T1-T2) temperature difference (K)
k =
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Definitions
Thermal conductivity of other common materials:
Material Thermal conductivity (298 K), W·m-1·K-1
Diamond 895-2300 ( the highest k)
Carbon Nanotubes
1400
Silver 429
Copper 386
Gold 317
Aluminium 237
Iron 80.2
BrickWood WoolStyrofoam(for building
insulation)
0.15 – 0.60.12 -0.040.04 (very low k)0.01
Why are diamonds so “cool”?
Why dose wool feel so “warm”?
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conversion
Thermal conductivity of other common materials:
Home Work: capacity of your heater and cooler?
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Phase change
Thermal conductivity of other common materials:
T-H Diagram
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